Neutrals and Electronics

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orion5284

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I was told that in a normal branch circuit (not multiwire/shared neutral), if the neutral at the receptacle/device is removed, the electronic equipment could be fried? To me, it seems that it simply would just open the circuit, and if ground is isolated at the device, current would stop because there is no more potential back to source. My understanding is voltage is a "potential" between two points. Since you are removing one of the points no voltage exists. It's only when you introduce another point, like earth/ground, would it seek back to source (secondary side) and thus have potential. If I were to grab a hot, and I'm completely isolated from anything, including ground, there would be no potential and no current.

So, back to the original question, if no potential (voltage), no current. So how would this damage any equipment? I've taken AC and DC theory, and I thought I understood all of this. But, apparently I don't.

Can anyone help me? I can't let things rest until I understand completely.
 
orion5284 said:
I was told that in a normal branch circuit (not multiwire/shared neutral), if the neutral at the receptacle/device is removed, the electronic equipment could be fried? To me, it seems that it simply would just open the circuit, and if ground is isolated at the device, current would stop because there is no more potential back to source. My understanding is voltage is a "potential" between two points. Since you are removing one of the points no voltage exists. It's only when you introduce another point, like earth/ground, would it seek back to source (secondary side) and thus have potential. If I were to grab a hot, and I'm completely isolated from anything, including ground, there would be no potential and no current.

So, back to the original question, if no potential (voltage), no current. So how would this damage any equipment? I've taken AC and DC theory, and I thought I understood all of this. But, apparently I don't.

Can anyone help me? I can't let things rest until I understand completely.
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You are correct and whoever told you that needs to have someone draw out a two wire circuit showing current flowing from and back to the source. A break in either conductor opens the circuit so current can not flow.

Roger
 
orion5284 said:
I was told that ......
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told by whom exactly?

if you grab a hot while standing on a block of rubber you will be the same potential as the hot.

voltage, regardless of magnitude or differential, has potential.

ron said:
Seems weird, when most servers have power supplies rated from 100V to 240V, autosense
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not all do, i seen a row of monitors pop because the new power strips came in with 240 and they swap the cords out. read the label is that story.
 
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Just talked to the guy again. He stated that there were four stations with computers. Each station had its own circuit and neutral, untapped, straight back to the neutral bar in the panel. No neutrals were shared. The guy thought power was off, and disconnected "one" neutral and fried every computer in each station?
 
Kinda related with the above scenario. Guy says that if you disconnect the hot at the device, circuit breaker on (closed circuit) and hold a volt tester (tick tracer) to the hot bare copper, it will show voltage on the line. Is that right, and why? Especially if you're completely not grounded. This I don't understand because there is no second point to create potential.
 
Can't see it happening.

Can't see it happening.

orion5284 said:
Just talked to the guy again. He stated that there were four stations with computers. Each station had its own circuit and neutral, untapped, straight back to the neutral bar in the panel. No neutrals were shared. The guy thought power was off, and disconnected "one" neutral and fried every computer in each station?
Click to expand...

There HAS to be more to the story than that. Was the one neutral he pulled the incoming one, that may do it!
It sounds like the neutral coming into the panel was bad and when he opened one of the four workstations he unbalanced the load, if the only thing on the panel was the four workstations.
 
Agree with others. There is more to the situation. It isn't wired the way the guy thinks it is. Someone probably swapped a neutral somewhere.
 
orion5284 said:
Just talked to the guy again. He stated that there were four stations with computers. Each station had its own circuit and neutral, untapped, straight back to the neutral bar in the panel. No neutrals were shared. The guy thought power was off, and disconnected "one" neutral and fried every computer in each station?
Click to expand...

If these are regular computers, they likely have a selector switch to be connected to 240V as was mentioned by texie. I thought the OP said servers.
 
This could happen pretty easily as matter of fact. . .and I've seen it more than once.
The computer station like most other 120/240 consumer utilization circuits are feed from secondary center tap transformer unlike some institutional or industrial setting from a 3 phase 120/208 wye.

Normally, for a center tap sourced distribution, the neutral is grounded. It is customary to bunch together neutrals at a junction.
If one of these bunched neutrals were disconnected and happens to be the homerun to the center tap you will get voltage reading from the two hot wire conductors which would be 240 volts. . . but you won't get the 120 volt to neutral because it is no longer connected to the center tap. Remember the electrician disconnected one neutral which could have been the homerun.

With all the downstream loads still connected, one hot lead would find a path to the source, which is of course another plugged-in computer--and the voltage would approach the 240 volts line potential of the transformer. It may be a little less than 240 caused by the impedance offered by the connected loads.

What I don't get is; why would the neutral be disconnected? Except in special applications like inverters where neutral is floating, we don't disconnect neutrals because of this scenario.

If you really must disconnect the neutral, make sure the whole group or main should be locked out.
Another reason why we don't route or switch neutral through a toggle switch.
 
orion5284 said:
Just talked to the guy again. He stated that there were four stations with computers. Each station had its own circuit and neutral, untapped, straight back to the neutral bar in the panel. No neutrals were shared. The guy thought power was off, and disconnected "one" neutral and fried every computer in each station?
Click to expand...

so in that great wisdom he traced the neutrals, but not the hots?

what exactly did he disconnect the "one" neutral from? a station, an outlet, in the panel ???
 
FionaZuppa said:
told by whom exactly?

if you grab a hot while standing on a block of rubber you will be the same potential as the hot.

voltage, regardless of magnitude or differential, has potential.
Click to expand...

Correct that you are at same potential as the "hot". Nothing happens to you as long as you remain insulated from other potential.

The only reason you have a "hot" or more then one "hot" on most multiwire systems is that another conductor of the system is connected to ground.
 
kwired said:
Correct that you are at same potential as the "hot". Nothing happens to you as long as you remain insulated from other potential.
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i hold the a 120vac hot that comes from breaker #1 in my teeth, then all of a sudden another 120vac hot shows up from breaker #3 (same pole), and i grab the 2nd one between my toes, but nothing happens even though that 2nd hot has plenty of potential. 120vac(rms)60Hz to be exact.
its a differential that is required to harm, but that alone is not good enough, its amps that kill :eek:hmy:.
 
FionaZuppa said:
voltage, regardless of magnitude or differential, has potential.
Click to expand...
I believe you know how this works, I just not sure how you worded it is all that clear to someone that may not understand.

Some graphics to go along with the explanation may save a lot of words
 
orion5284 said:
I was told that in a normal branch circuit (not multiwire/shared neutral), if the neutral at the receptacle/device is removed, the electronic equipment could be fried? To me, it seems that it simply would just open the circuit, and if ground is isolated at the device, current would stop because there is no more potential back to source. My understanding is voltage is a "potential" between two points. Since you are removing one of the points no voltage exists. It's only when you introduce another point, like earth/ground, would it seek back to source (secondary side) and thus have potential. If I were to grab a hot, and I'm completely isolated from anything, including ground, there would be no potential and no current.

So, back to the original question, if no potential (voltage), no current. So how would this damage any equipment? I've taken AC and DC theory, and I thought I understood all of this. But, apparently I don't.

Can anyone help me? I can't let things rest until I understand completely.
Click to expand...

Welcome to the forum.

A MWBC can be effectively split into 2 or 3 branch circuits. If you lift the neutral at the branch circuit, nothing happens. If you lift it at the MWBC portion, that's when stuff can get fried.

If you grab a hot but are not grounded, you still have potential, tho no current flow. Bird on a wire. Eagle lands on a 20kV overhead line, nothing happens. If he takes off and his wing hits another conductor of the overhead 3 phase service, well, it's not pretty.
 
JFletcher said:
Eagle lands on a 20kV overhead line, nothing happens. If he takes off and his wing hits another conductor of the overhead 3 phase service, well, it's not pretty.
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bird can also die just from landing on the one 20kv wire. if the bird is not at 20kv when touching the wire the bird-to-wire potential diff will create a small current. the bird is a small cap at that point.

the transfer of electrons to balance the charge can kill a bird that lands on 20kv, and many other things, including people.

you can see the bolt jump to the rod, and there is a reason why he uses a rod and not his hand ;)

right at ~2:09 (that nice bolt of electrons flow. and notice no tools or equip are tethered :eek:hmy:)

not sure why it does not embed correctly using time marker in URL (hint to site admins), you can also use this link to jump right to the mark
https://www.youtube.com/watch?v=DPNK7bc2qvM#t=2m07s

 
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